1. Field of the Invention
The present invention relates to a light receiving circuit, more particularly, to a light receiving circuit that converts a photocurrent generated in a photodiode to a voltage signal to output the generated voltage signal.
2. Description of Related Art
In recent years, an optical disk device capable of writing data to an optical disk medium (CD (Compact Disk) or DVD (Digital Video Disk) and soon) and reading data from the optical disk medium has been widely used. Inside the optical disk device, a photodiode is provided for converting a reflected laser light, which is used for reading/writing data, to a photocurrent. This current signal generated by the photodiode is converted to a voltage signal by an IV converter (equivalent to a current-voltage conversion circuit). This generated voltage signal is output from the IV converter.
When the light receiving circuit is used in the optical disk device for two different standards of CD/DVD, it is required for the light receiving circuit to be capable of switching two photodiodes. This is because a two-wavelength laser is often used in the optical disk device for two different standards of CD/DVD.
A light receiving circuit capable of switching two photodiodes is disclosed in Japanese Unexamined Patent Application Publication No. 2006-109434 (hereinafter referred to as prior art 1) as shown in FIG. 6.
As shown in FIG. 6, two branches are connected to a common IV converter. Each of the branches (branch circuits) has a photodiode, a first switch SW1, and a second switch SW2. The photodiode and the first switch SW1 are connected in series at each branch. The second switch SW2 is connected to a node between the photodiode and the first switch SW1 at each branch. The first switches SW1 are connected to the IV converter in parallel.
In this light receiving circuit 60, a complementary operation is performed between the first and second switches at each branch. That is, the first switch SW1 is in ON state when the second switch SW2 is in OFF state, and the first switch SW1 is in OFF state when the second switch SW2 is in ON state. By adopting this complementary operation, an undesired influence on the IV converter which is caused when a light is input to the photodiode that is not selected (hereinafter also referred to as an ineffective photodiode) is avoided.
A trans-impedance amplifier disclosed in Japanese Unexamined Patent Application Publication No. 2002-84149 (hereinafter referred to as prior art 2) is shown in FIG. 8. As shown in FIG. 8, the trans-impedance amplifier 80 has an amplifier AMP having feedback resistors R9 and R10 connected in series between input and output terminals thereof. The amplifier AMP converts an input current from a photodiode connected to the input terminal thereof to a voltage and outputs the generated voltage from the output terminal thereof.
In this trans-impedance amplifier 80, a negative feedback current is supplied to a signal input terminal side of the amplifier AMP when the input current generated in the photodiode PD becomes equal to or above a predetermined level. More specifically, an operational point for turning on an NPN transistor Tr1 connected to VCC and an output clipped voltage are determined by a resistance ratio between resistors R9 and R10. By adopting this configuration, stable and safe operation is achieved even if a large amount of photocurrent is generated in the photodiode PD.
Still though, it is not achieved to solve problems explained below simultaneously by the prior art 1 or the prior art 2 in case that a plurality of photodiodes are provided in the light receiving circuit. In the prior art 1, a cathode potential of the photodiode lowers when a photocurrent over a certain level enough for saturating the output voltage of the IV converter is input to the IV converter. In addition, a certain period of time is required for recovering the cathode potential, and a delay is caused in a recovery characteristic of the output voltage of the IV converter when the input photocurrents are switched as shown in FIG. 7.
On the other hand, in the prior art 2, the undesired influence is applied to the IV converter which is caused by the photocurrent that is generated when the light is input to the ineffective photodiode.
As explained above, the undesired influence on the IV converter caused by the over-saturated state and the undesired influence on the IV converter caused by the photocurrent generated when the light is input to the ineffective photodiode have not been solved simultaneously.